Rangemaster Profesional+ Gas 110 – Ovens fire up and then cuts out,when hot and cold?

Hello, i have a Rangemaster Prof Gas 110, its about 5 ys old, in excellent condition. The oven very often when hot will fire up for a few seconds, then the flame cuts out, it will try again and then cut out. We tried the other oven tonight that was cold and the same, whilst continually clicking to ignight the flame cuts out. Maybe by chance i tried filming it with teh door open and it ignited and stayed ignited , job done. Most of the local Gas repairers seem to have retired and wondered if anyone had any ideas please? many thanks Els